Such a LED arrangement is known from U.S. Pat. No. 5,959,413. U.S. Pat. No. 5,959,413 discloses a driving circuit in which each LED has a controllable logic switch in parallel across it and the switches are further in series circuit with each other to form a ladder network. Any selected LED may be switched off by closing its corresponding switch. The current continues to flow then through the shunting switch into the remaining LEDs in the series circuit that are on. A plurality of such ladder networks may be coupled in parallel with each other and each ladder network may be controlled by a switching gate which selectively couples it to the constant current source so that the LED ladder networks are operated at a predetermined duty cycle. Current spikes are avoided across the voltage supply by driving the connecting control gates of the parallel strings in an overlapping relationship so that the constant current source is never disconnected from the voltage supply.
The known circuit has the disadvantage that it is required to be controlled in such a way that always a LED is driven to prevent current spikes in the power supply line. Hence it is needed to use an overlapping driving scheme for the parallel strings and it is needed to distribute all LEDs over a plurality of strings if a low duty cycle is required. This adversely limits the range of duty cycles that can be used when operating the LEDs.
An alternative arrangement is known from US patent application US2005/0243022 A1. An efficient power supply in the form of a switched-mode power supply is provided in FIGS. 6 and 7 of US2005/0243022 A1. The switched-mode power supply uses a switch, a coil and a diode, where the switch is operated to charge the coil, which is discharged via the diode. In such an arrangement, the current shows a large ripple, i.e., it fluctuates with a large amplitude around an average level. A known solution to limit this ripple to a relatively small amplitude is to place a filter capacitor over the output of switch mode supply. A disadvantage of this approach is that current spikes occur when the load on the switch mode is changing, as a result of switching LEDs on and off in the series arrangement. The current spikes can damage the LEDs as well as the power supply.